This invention is related to protective coatings for phosphor particles. More particularly, this invention relates to aluminate phosphors such as barium magnesium aluminate activated by Eu2+ and coatings applied thereto.
Common aluminate phosphors include (Ce, Tb)MgAl11O19:Ce3+,Tb3+, Sr4Al14O25:Eu2+ and BaMgAl10O17:Eu2+. Such phosphors are widely used in fluorescent lamps and more recently in electronic display devices such as plasma display panels (PDP). The ability of these phosphors to resist losing brightness during the manufacture and operation of these devices is an important consideration. For example, the brightness of the widely used blue-emitting barium magnesium aluminate phosphor, BaMgAl10O17:Eu2+, (BAM) degrades during the thermal processing and operation of fluorescent lamps and PDP devices. The disproportionate decrease in the light output from the blue-emitting BAM phosphor component causes an undesirable color shift in the overall emission from these devices.
The thermal degradation of the phosphors during thermal processing is believed to be related to oxidation of the activator ions. In the case of BAM phosphors, oxygen atoms are believed to diffuse into the BAM phosphor to the vicinity of the Eu2+ ions and oxidize them. The ease with which oxygen atoms or other large ions diffuse in the BAM lattice is related to its open structure in the intermediate plane between blocks of spinel-like structures. The high temperature processing of fluorescent lamps and PDP displays promotes this oxidative degradation leading to an initial loss in lumens. The radiative degradation which occurs over time in mercury-discharge lamps is believed to be primarily due to color center formation caused by mercury""s energetic vacuum-ultraviolet emission at 185 nm. This causes a further decrease in lumen output from the phosphor during the operation of fluorescent lamps.
It is an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to provide a protective coating for reducing the thermal and radiative degradation of aluminate phosphors.
It is yet another object of the invention to provide a method for applying a protective coating which is compatible with conventional fluorescent lamp manufacturing techniques.
In accordance with one object the invention, there is provided an aluminate phosphor having at least a partial spinel coating formulated to have a general formula Mg1-xAl2(1-y)O4-3y-x, where 0xe2x89xa6x less than 1 and 0xe2x89xa6y less than 1.
In accordance with another object of the invention, there is provided a method of applying at least a partial spinel coating on an aluminate phosphor comprising the steps of:
(a) combining an aqueous solution of coating precursors and an aluminate phosphor, the aqueous solution formulated to yield a spinel coating having a general formula Mg1-xAl2(1-y)O4-3y-x, where 0xe2x89xa6x less than 1 and 0xe2x89xa6y less than 1;
(b) drying the aqueous solution containing the phosphor; and
(c) firing the phosphor to form the at least partial coating.
For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims.
We have discovered that the thermal and radiative degradation of an aluminate phosphor, in particular, the blue-emitting barium magnesium aluminate (BAM) phosphor, BaMgAl10O17:Eu2+, may be controlled by applying at least a partial spinel coating on the phosphor surface. The coating is applied by an aqueous solution method which may be compatible with current fluorescent lamp manufacturing techniques. The spinel coating is formulated to have the general formula Mg1-xAl2(1-y)O4-3y-x, where 0xe2x89xa6x less than 1 and 0xe2x89xa6y less than 1. In a preferred coating composition, the values for x and y are x=0 and 0xe2x89xa6y less than 2/3. More preferably, x is 0 and y is about 0.5.
The spinel coating of this invention is believed to provide a diffusion barrier which reduces the oxidation of the activator ions. Known oxygen diffusion coefficients in materials having a xcex2-alumina structure indicate that it is more difficult for oxygen ions to move through the spinel layers of xcex2-alumina compared to diffusion along the intermediate planes of the material. Furthermore, the spinel coating contains ions already present in some aluminate phosphors, particularly BAM. As a result, the diffusion of these ions (or its significance) between the bulk phosphor and the coating during the coating process may be reduced compared to phosphors lacking aluminum and magnesium ions. Ideal spinel, MgAl2O4 is known to be transparent to the primary 254 nm UV radiation from a mercury discharge and has a relatively dense anion packing. The transparency of the spinel coating of this invention to 254 nm radiation allows the underlying phosphor to be excited by the primary UV radiation from the Hg-discharge. Furthermore, the ternary spinel composition may be varied over a wide range in order to tailor the properties of protective coating for the specific aluminate phosphor.
In a general method, the spinel coating of this invention is applied by combining the aluminate phosphor with an aqueous solution of the coating precursors which is formulated to yield the desired coating composition. The solution is then dried and the phosphor is fired to form the coating. In a preferred method, the coating precursors are nitrates and the dried phosphor is first fired in air at a lower temperature (preferably, 600xc2x0 C. for 30 mins) to remove the nitrate groups and then in a reducing atmosphere at a higher temperature (preferably, 1200xc2x0 C. for 90 mins). In a more preferred method, the nitrates are dissolved into a minimum amount of water at about 90xc2x0 C., stirred and finally mixed with the phosphor powder in a 10% or higher weight ratio. The slurried phosphor is thereafter dried at about 120xc2x0 C. in air for a few hours and oxidized at 400xc2x0 C. in air for 2-3 hours. After manually breaking up the agglomerates, the phosphor powder is subjected to a flash firing procedure performed in forming gas (5% H2 in N2) at about 900xc2x0 C. for 5 minutes. The resulting powder is manually ground for regaining the initial approximate particle size.
The beneficial influence of the spinel coating of this invention in reducing the thermal and radiative degradation of a BAM phosphor is demonstrated in the following non-limiting examples.